1. Field of the Invention
The present invention relates to a thin film transistor.
2. Description of the Related Art
Most of display units such as a liquid crystal display, an organic EL display, and an electrophoretic type display are provided with a thin film transistor (hereinafter, also referred to as “TFT”) as a display switching device therein. The TFT has a structure including a gate electrode, a semiconductor layer, and a gate insulating layer provided between the gate electrode and the semiconductor layer on a substrate, and is also provided with a source electrode and a drain electrode in contact with the semiconductor layer. The TFT is driven by applying a voltage to the gate electrode. A driving principle of the FET is to control an amount of carriers consisting of electrons or holes in a semiconductor by applying a voltage to the gate electrode and thereby control an electric current flowing between the source electrode and the drain electrode.
An inorganic semiconductor such as amorphous or polycrystalline thin film silicon has been conventionally used as a semiconductor for TFTs. In the case where a semiconductor layer of TFTs is formed of an inorganic semiconductor, a vacuum process and a high temperature process at 300° C. or higher are required and therefore there is a limitation on improvement of productivity.
To cope with this, TFTs using an organic semiconductor also have recently become popular. Because an organic semiconductor layer can be formed into a film by a method such as an ink jet method, a spin coating method, or a flexographic printing method, a film forming process can be performed at a lower temperature, at high speed with efficiency, and at low cost.
Most of TFTs using an organic semiconductor in the semiconductor layer use silicon oxide obtained by thermal oxidation of silicon as a gate insulating layer. In the case of using a silicon oxide film, a silicon oxide film surface is typically treated with hexamethyldisilazane (HMDS), octadecyltrichlorosilane (OTS), or the like to render the surface water-repellent, in order to allow the carrier conducting performance of an organic semiconductor to be formed on the silicon oxide film to be sufficiently exhibited. Since hydroxyl groups of the silicon oxide film surface are quenched and further, the surface energy of the gate insulating film is lowered to improve crystallinity of the semiconductor by such a surface treatment, the carrier mobility is improved. However, there is a case where HMDS and OTS aggregate or polymerize on the surface of the gate insulating film. If that is the case, it becomes difficult for the semiconductor to grow into a crystal, and therefore sufficient carrier mobility is not obtained.
Further, a TFT where the gate insulating film is formed of an organic polymer compound is also known. For example, JP2006-303465A discloses a TFT where a gate insulating layer is formed of a cured product formed by using a resin and a crosslinking agent. JP2006-303465A discloses that it is desirable to prevent contamination of an organic semiconductor layer by reducing the content of an alkali metal in the gate insulating layer and additionally that an amount of sodium is preferably set to 20 ppm or less.